This invention relates to a process for polymerizing unsaturated hydrocarbons of the formula
CH2xe2x95x90CHR
in which R is a saturated aliphatic, an alicyclic or an aromatic radical, alone, in mixture with one another, or in mixtures with small amounts, up to about 5%, of a monomer polymerizable therewith.
Uniquely, the initial polymerization products obtained by the present method are mixtures of linear, head-to-tail amorphous and crystalline polymers having no branches longer than R. The polymers can be separated from the polymerizate by fractional dissolution. The crystalline polymers may comprise as nigh as 30% or even up to 55% of the mixture and have high molecular weights and fiber forming properties. The amorphous polymers may also have relatively high molecular weights and may exhibit rubber-like properties.
It has been proposed, in the Belgian patent No. 533,362, to polymerize ethylene to polymers of high molecular weight of the order of 300,000 up to 2,000,000 or more, by using as reaction initiators, agents obtained by reacting a catalytic heavy metal compound and a catalytic metal alkyl compound.
The catalytic heavy metal compounds we use in preparing the catalyst for the polymerization of the alpha-olefins to polymers having the steric structures described below are halides of heavy metal selected from the sub-group of Groups IV to VI of the Periodic Table, including thorium, and uranium, i.e., halides of the elements of titanium, zirconium, hafnium, thorium, vanadium, tantalum, niobium, chromium, molybdenum, tungsten and uranium. These are metals belonging to Groups IVa, Va and VIa of the Mendeleeff Periodic Table.
The catalytic metal alkyl compounds we use in preparing our catalyst are alkyl compounds of elements selected from the group forming the 2nd and 3rd columns of the Periodic Table, i.e., beryllium, magnesium, zinc, cadmium and other elements of the 2nd group, as well as boron, aluminum and other elements of the 3rd group.
The valences of the aforesaid elements are linked to the same or different alkyl radicals such as ethyl, propyl, butyl, etc. One valence of the element may be satisfied by halogen, or alkoxy radicals.
Although, as stated, the polymerization aids obtained by reacting heavy metal compounds and metal alkyl compounds in a solvent inert to the polymer to be formed, such as a saturated aliphatic hydrocarbon, were found useful in the production of high polymers of ethylene, it was not apparent that those agents would be useful in the polymerization of the unsaturated hydrocarbons containing the vinyl group.
Processes and polymerization promoters that are useful for producing polyethylene of high molecular weight are not necessarily useful for producing high molecular weight polymers of the higher homologues of ethylene, such as, for instance, propylene.
In the past, it has been found that when various promoters or catalysts useful in producing high molecular weight polyethylene have been used with the higher homologues, for instance only dimers, trimers or tetramers forming light oils, or somewhat higher polymers comprising more or less viscous lubricating oils.
The products obtained by polymerizing the higher ethylene homologues by processes known in the art generally consist of mixtures of polymeric homologues containing variously branched isomers. The higher the temperature used, the smaller the degree of polymerization. But even when the known processes are carried out at relatively low temperature the products obtained are still mainly liquids or mixtures of liquids and amorphous products in which the liquid component is present in substantial amount.
Prior to this invention, it has been considered that polymerization of olefins CH2xe2x95x90CHR, i.e., olefins in which one of the hydrogen atoms of a CH2 group is replaced by a CH3 group, such as propylene, results in non-crystalline polymers. Thus, it has been stated in the literature (xe2x80x9cFibers from Synthetic Polymersxe2x80x9d, Rowland Hill, Ed., Elsvier Publishing Co., 1953, p. 319) that:
xe2x80x9cPolymers of olefins in which one of. the hydrogen atoms of a CH2 group is replaced by a CH3 group, are usually non-crystalline, owing to stereochemical irregularity.xe2x80x9d
It was believed, further, that the low crystallinity exhibited by polyvinyl chloride for example, was due to the fact that over some short sections of the polymer main chain the configurations of the tertiary carbon atoms of the CHC1 groups are alternately levo and dextro.
Furthermore, it was assumed that, since the methyl group is similar in size to the chlorine atom, it was to be expected that polymers like polypropylene in which one could assume that tertiary asymmetric carbon atoms of the main chain having different steric configurations would occur in a random distribution, would be non-crystalline, owing to the stereochemical irregularity. (xe2x80x9cFibers Prom Synthetic Polymersxe2x80x9d, supra, p. 235, Hill and Walker, Jr. of Polymer Science, Vol. III, 1948, p. 609).
It has been found, also, that even when ethylene is polymerized with the aid of the special promoters obtained by reaction of heavy metal and metal alkyl compounds, in the ratios proposed for promoting ethylene.
The slowness to polymerize exhibited by the higher olefins is not overcome by merely increasing the polymerization temperature, since when the special aids described herein are used, temperatures which promote the growth of the polymer chain, bound in the form of a metallo-organic compound, without appreciably favoring its disassociation into low molecular weight olefins must be used.
In view of the foregoing, it could not be predicted, from the work with ethylene, that our polymerization agents would be useful for the production of higher molecular weight polymers of the vinyl hydrocarbons of formula CH2xe2x95x90CHR as defined herein.
Surprisingly, we have now foud that, in fact it is possible to obtain high molecular weight polymers of propylene and other monomeric vinyl hydrocarbons of that type, i.e., containing more than three carbon atoms per molecule and including styrene, and to produce the polymers at commercially acceptable reaction rates and in good yields, with the aid of the aforesaid initiators if the initiator is prepared from the heavy metal halide and metal alkyl compound in appropriate molar ratios.
In accordance with the invention, the new polymerization products are obtained by polymerizing monomeric material comprising at least one of the vinyl hydrocarbons containing three or more carbon atoms per molecule in the presence of polymerization aids obtained by reacting the heavy metal halide and metal alkyl compound in molar ratios such that the metal alkyl component is not more than ten times the heavy metal halide and is preferably less than five times the heavy metal halide, as will be seen from the examples given hereinafter. The catalyst is the reaction product of (a) a chloride of titanium with (b) an alkyl aluminum compound in which at least two of the valences of aluminum are satisfied by alkyl groups, and in which the third valence can be satisfied by alkyl or halide. The catalyst may also be the reaction product of (a) a chloride of titanium with (b) an aluminum trialkyl compound, in which the alkyl groups have up to four carbons each. To obtain a highly active catalyst, and effect smooth polymerization of the olefin, it is important to carry out the reaction between the heavy metal halide and the metal alkyl compound in an inert solvent preferably a saturated hydrocarbon solvent. This insures that the reaction proceeds smoothly, without violence, and without a strong temperature increase which would be considerable if the catalyst were used without prior dilution thereof.
Solvents suitable for use in preparing the polymerization catalyst are paraffinic hydrocarbons such as, for instance, a light gasoline, (substantially free of olefinic bonds), n-heptane, iso-octane, and other substances preferably of the non-aromatic type. However, benzene has also been used.
The solvents selected should preferably have boiling points lower than 150xc2x0 C. and not swell the highest molecular weight polymers obtained.
The heavy metal halide and metal alkyl compound may be dissolved separately in a solvent of the same group, and the solutions then brought together maintaining a low temperature, preferably a temperature somewhat below room temperature, such as 0xc2x0 C. to 20xc2x0 C.
It has also been found advantageous, in some instances, to add a small amount of isopropyl alcohol to the solution of the metal alkyl compound in one of the aforesaid solvents before adding the solution of the heavy metal halide to it.
Preferably, the solution of the heavy metal halide is added dropwise to the cooled solution of the metal alkyl compound.
The composition thus obtained may be further diluted with solvent and introduced into an autoclave for instance of the oscillating stainless steel type.
The autoclave should be previously thoroughly dried and evacuated because it is of general advantage to carry out the entire process in the absence of oxygen and water.
The presence of oxygen and water should be limited to small quantities, i.e., impurities, because they will consume a part of the metal alkyl component.
The vinyl hydrocarbon or mixture containing it is preferably dried carefully and then pumped into the autoclave either in the liquid or gaseous phase, or as a mixture of both phases.
In the autoclave, the monomeric material is heated to a temperature below 120xc2x0 C., and preferably to a temperature between 50 and 100xc2x0 C. The pressure applied in the autoclave is preferably held between normal atmospheric pressure and 30 atmospheres, i.e., a relatively low pressure is used compared with that applied in other polymerization processes.
The polymerization can be carried out at a temperature between 50 and 100xc2x0 C. and more specifically between 60 and 70xc2x0 C. for the polymerization of propylene.
The upper limit for the temperature range is determined by the fact that at temperatures substantially above 100xc2x0 C.
During the polymerization step in the autoclave the pressure falls gradually with progressing polymerization. The vinyl hydrocarbon or mixture may be pumped in one or several batches.
The polymerization lasts for several hours up to several days, the autoclave being kept in motion until the decrease of pressure ceases. The gaseous phase above the reaction product is then vented, and the reaction product consisting of a solid mass is worked up.
The reaction mass contains, as impurities, inorganic compounds originating from the decomposition of the catalyst as well as residual catalyst itself. The product is, therefore, treated with a suitable agent, for instance methanol, for decomposing the residual catalyst, and the product which is still soaked with the inert solvent and, say, methanol, then contains
(a) isotactic (crystalline) polymers;
(b) partially isotactic polymers (such as the partially crystalline fraction described in Examples 1 and 2 below);
(c) non-isotactic or atactic (amorphous) polymers;
(d) inert solvent;
(e) methanol or the like; and
(f) inorganic compounds, e.g., of Al and Ti.
This mass may be treated with isopropyl ether to dissolve the amorphous polymers; a dark suspension of the crystalline polymer including the inorganic compounds remains. By bubbling HCl through, the inorganic compounds are dissolved and the suspension becomes white. (The crystalline polymer remains undissolved.) By adding additional methanol, most of the dissolved amorphous polymer is precipitated. The purified polymer mixture is separated by filtration from the solvent mixture.
The initial polymerization product is, as has been stated, a mixture of polymers. It comprises, usually, a small amount of an oily low molecular weight fraction, an amorphous fraction of higher molecular weight, and a high molecular weight fraction, the amorphous and crystalline polymers making up the bulk, generally, of the mixture.
The products are thus mainly made up of polymers that can be classified in two types, not crystallizable, amorphous and crystalline. The mixtures can be used as such for various purposes, or the different fractions can be separated depending on their solubility in various solvents; the fractions varying in their amorphous and crystalline contents, in average molecular weights, and specific viscosity.
The molecular weight of the products was estimated from specific viscosity measurements in tetrahydro-naphthalene solutions at a polymer concentration of 0.1 gm. per 100 gms. of solvent and from intrinsic viscosity measurements. Specific viscosity is the viscosity of the solution less the viscosity of the solvent, divided by the viscosity of the solvent. By xe2x80x9cintrinsic viscosityxe2x80x9d is meant the limit of the ratio between specific viscosity and concentration for concentrations tending to zero   (                    Limit                  C          ⟶          0                    ⁢              xe2x80x83            ⁢                                    μ            spec                    ⁢                      xe2x80x83                          C              ,  
where
C is the concentration of the solution in gms/cs.).
Products having an average molecular weight above 20,000 and up to 400,000 and higher may be obtained. The products are initially, and as has been stated, mixtures of polymers.
These mixtures comprise polymers which, depending on their steric structure, and their molecular weight, have very different characteristics. The amorphous products have viscous elastic properties comprised between those of a highly viscous liquid and those of an unvulcanized, non-crystallizable elastomer, while the solid, highly crystalline products, which can be oriented by drawing, give products of fiber-like behaviour. Both types of polymers are, as shown by their infra-red spectra, linear; in the case of polypropylene, e.g., both amorphous and crystalline polymers have similar infra-red spectra which are completely different from the infra-red spectra of the known branched polypropylene in which the branches are longer than R.
The difference in the properties of the two types of polymers which we have obtained, must be attributed to a different distribution, along the main chain, of those asymmetric carbon atoms having the same steric configuration.
According to Flory (Principles of Polymer Chemistry, 1953, pg. 55-56, 237-38) a vinyl polymer containing asymmetric carbon atoms, as e.g. 
may be considered as a copolymer of two different monomer units 
in one of which the asymmetric C atom has a 1 configuration, and in the other a d configuration.
When such monomer units containing an asymmetric carbon atom showing a 1 or d configuration recur statistically along the polymer chain, as in general in all known vinyl polymers, the polymer may be considered as a copolymer of the two types of structural units, and therefore, if the substituent R is much larger than a H atom, is substantially non-crystalline and does not show any 1st order transition point.
Up to now the only known example of a vinyl polymer existing in an amorphous and in a crystalline form are the polyvinyl ethers prepared by Schildknecht and co-workers (Ind. Eng. Chem. 40 (1948) 2104, ibid. 41 (1949) 1998, 2891). The structure of the new crystalline high polymers of the alpha-olefins obtained by the present process was determined from x-rays data on drawn fibers of said polymers.
The elementary cell dimensions for the different alpha-olefine polymers were measured as reported in the accompanying Table I.
From the above it is clearly apparent that the identity period along the fiber axis is, in all cases, of the order of magnitude of 6.5-6.7 xc3x85.
By comparing X-ray and density data, it may be seen that each stretch of principal chain included in the elementary cell corresponds to 3 monomeric units (xe2x80x94CH2xe2x80x94CHRxe2x80x94), and, that therefore a regular succession of monomeric units having alternatively d and 1 asymmetric carbon atoms can be excluded. Among all possible remaining regular successions of d and 1 asymmetric carbon atoms which could lead to a crystalline polymer it is believed on the basis of the X-ray data, that the most probable is the one in which, at least for long portions of the main chain, all the asymmetric C atoms have the same steric configuration: 
In this case the stable existence of a planar fully extended paraffinic chain seems most unlikely, owing to the steric hindrance of the substituent groups R. In the crystalline state, the main chain must therefore assume a non-planar conformation. This conformation has been found to be spiral-like.
The hypothesis of a coiled conformation of the main chain in the crystalline state agrees with the value of the identity period along the same chain (6.5-6.7 xc3x85) which is smaller than the length of the planar, fully extended structure (7.62 xc3x85 for 3 monomeric units).
Our linear, regular, head-to-tail macromolecules having substantially no branches longer than R and the main chain of which has substantially a structure of the kind illustrated in the model (isotactic structure) are recognized in the art (following us) as xe2x80x9cisotacticxe2x80x9d macromolecules, whereas our macromolecules having
The term xe2x80x9cisotacticxe2x80x9d was originated by one of us, G. Natta,
The partially isotactic polymers produced and isolated from the crude polymerizates by us, such as the partially crystalline fraction described in, for instance, Examples 1 and 2 below, comprise macromolecules consisting of sections having the Natta isotactic structure and sections having the Natta non-isotactic or atactic structure.
The isotactic structure imparts to the product properties not previously known for any hydrocarbon. In fact these products with a high molecular weight are, at room temperature, crystalline solids very different, e.g., from the elastomers obtained by known methods from isobutylene, which are crystallizable under stress and show a spiral-like linear chain, but with a different identity period. The substantial differences in the physical properties of the two types of polymers prepared by the present process (amorphous and crystalline) are summarized in Table 2.
The very remarkable differences in the solubilities of the two types of polymer permit of their easy separation by extraction with suitable solvents. For a given polymeric alpha-olefin the crystalline products always show a much lower solubility than the amorphous products independently from the molecular weight, while the solubilities of polymers of the same type, but having different molecular weight, decrease slightly and gradually with an increase of the molecular weight.
When examining, e.g. the fractions obtained by fractional dissolution of the propylene polymers as described in Example I below, that portion of the product which may be extracted with ether (intrinsic viscosity 1) appears wholly amorphous under the X-rays. The further extracts in boiling n-heptane (intrinsic viscosity of extracted polypropylenes 1.2-1.3, average molecular weight probably within the range 45,000 to 55,000), appear already partially crystalline under the X-rays and melt completely at 150xc2x0 C.
It was found that the polypropylene products which are insoluble in boiling n-heptane and having a higher molecular weight than those previously mentioned, probably above 100,000 (intrinsic viscosity above 3), are solids having a density between 0.90 and 0.94, and a high melting point (about 160xc2x0 C). A density of about 0.94 is the upper limit for a completely crystalline product. Said density was calculated from reticular distances determined from X-rays diffraction patterns of fibers. The specific gravity is lowered by the presence of an amorphous portion and in fact the amorphous ethersoluble polymers have a density lower than 0.90. Above the temperature at which the crystalline state disappears, the polypropylene products do not change directly into a flowing liquid, but retain the characteristics of an amorphous plastic substance. They can, therefore, be readily processed (at a temperature somewhat higher than the temperature of complete disappearance of the crystalline structure) by the methods (moulding, injection, extrusion) known for plastic materials. It is found, moreover, that these propylene polymers of Example I of very high molecular weight, with intrinsic viscosity above 3, having a high degree of crystallinity which is preserved up to relatively high temperatures (above 150xc2x0 C.), show surprising mechanical characteristics, as will be further described, not previously accertained for any known hydrocarbon so that they can be satisfactorily employed in making textile fibers.
The propylene polymers with very high molecular weight obtained by the instant method may be molded at temperatures higher than 130-150xc2x0 C. to obtain plates, which are transparent when the product is well purified. Such plates may be cold stretched up to about 700%; the breaking load, referred to the section resulting after stretching, may be, e.g. for the product obtained according to Example II, higher than 30 kg/sq. mm., and even considerably higher if the product is drawn into thin filaments. The stretched filaments show a silk-like appearance and their good mechanical characteristics (high breaking load and high elongation at rupture of the stretched filament) make them particularly suitable for producing textile fibers.